1. Field of the Invention
The present invention relates to a fluorescent lamp and a method for manufacturing the same, more particularly to a fluorescent lamp and a method for manufacturing the same, which is constructed such that an electric discharge is performed through a tubular glass bulb or a dielectric between an internal electrode axially passing through the tubular glass bulb forming a discharge chamber and an external electrode provided on an outer surface of the tubular glass bulb.
2. Background Art
An example of a prior fluorescent lamp, such a type as described above, constructed such that a discharge is performed through a dielectric between an internal electrode and an external electrode is disclosed, for example, in Japanese Patent Laid-Open Publication Hei. 7-272694.
FIG. 10 shows an example of a constitution of a prior fluorescent lamp 80 such a type as described above. A tubular glass bulb 81 applied a fluorescent layer 82 on an inner surface thereof has been discharged the air, filled with gas and then sealed at both ends, thereby forming a discharge chamber 81a. An external electrode 83 is formed on an outer surface of the tubular glass bulb 81.
In the discharge chamber 81a, an internal electrode 84 made of a metal rod or a thin and long metal tube is laid substantially along the central axis of the tubular glass bulb 81.
FIG. 11 shows another example of a constitution of a prior fluorescent lamp 85 such a type as described above. A tubular glass bulb 87 applied a fluorescent layer 82 on an inner surface thereof has been sealed by flare stems 88a, 88b at both ends, thereby forming a discharge chamber 89. In the discharge chamber 89, a getter 90 and an internal electrode 91 are provided, and rare gas (not shown) is filled. An external electrode 92, made of a conductive coating or the like, is formed on an outer surface of the tubular glass bulb 87. A light emitting is accomplished by applying high-frequency power between the internal electrode 91 and the external electrode 92.
However, there are some problems with the above-described fluorescent lamp constructed such that a discharge is performed through a dielectric, compared to a conventional fluorescent lamp, which has been employed, constructed such that a pair of electrodes face each other and a discharge is performed only in a discharge chamber.
In the above-described fluorescent lamp, it has a constitution that an internal electrode axially passing through a tubular glass bulb. Therefore, it causes a first problem that the internal electrode or the tubular glass bulb is deformed or broken because of a change in the ambient temperature, a temperature rise in discharging or the like, since the coefficient of expansion of the internal electrode made of a metal differs from that of the tubular glass bulb.
Also, the discharge is performed between almost whole of the surface of the internal electrode and the external electrode. Therefore, for example, when a part where a discharge resistance is slightly lower than another part exists in the electrodes, the discharge tends to be concentrated on the part, which in turn causes a second problem that the concentrated part in the internal electrode or the external electrode is exhausted.
In addition, the discharge is performed through a tubular glass bulb as a dielectric. Therefore, the tubular glass bulb is also exhausted. Particularly when the discharge is concentrated as described above, the concentrated part of the tubular glass bulb is holed to lose functions as the discharge chamber, which in turn causes a third problem that the whole of the fluorescent lamp is broken.
Furthermore, the fluorescent lamp of this type causes a fourth problem that it shows a tendency to be a lower efficiency compared to the above-mentioned conventional fluorescent lamp constructed such that a discharge is performed only in a discharge chamber. Accordingly, with regard to a fluorescent lamp constructed such that a discharge is performed through a dielectric, unique problems, which will be overcome by the present invention, arising from such construction occurs.
A method for manufacturing the conventional fluorescent lamp 85 shown in FIG. 11 is next described in order of process in connection with FIGS. 12A to 12D. First, as shown in FIG. 12A, a mount is formed by connecting a flare stem 88a, to which a getter 90 is attached, to one end of the internal electrode 91 made of, for example, a tungsten wire by spot welding or the like.
Next, as shown in FIG. 12B, the mount is inserted into the tubular glass bulb 87 in a condition that the internal electrode 91 is hung downward. The flare stem 88a is deeply inserted so that the other end of the internal electrode 91 appears at the bottom of the tubular glass bulb 87. The other flare stem 88b is attached to the other end of the internal electrode 91 by spot welding or the like. Furthermore, a fluorescent layer 86 is formed on the inner surface of the tubular glass bulb 87 in advance.
Next, as shown in FIG. 12C, the flare stem 88a is pulled up so that the position thereof agrees with the upper end of the tubular glass bulb 87, and the flare stem 88a is welded. Thereafter, as shown in FIG. 12D, the other flare stem 88b is positioned so that the position thereof agrees with the lower end of the tubular glass bulb 87, and the other flare stem 88b is welded.
However, according to this conventional manufacturing method, the flare stem 88a is deeply inserted into the tubular glass bulb 87 in order to attach the other flare stem 88b, as shown in FIG. 12B. Therefore, the flare stem 88a attains the fluorescent layer 86, thereby causing a problem that the circumference of the flare stem 88a scratches the fluorescent layer 86.
Additionally, the above manufacturing method contains steps, for example, such that the flare stem 88a is made to have a round trip in the tubular glass bulb 87 so as to attach the flare stem 88b, which is troublesome and is not always necessary for the completed fluorescent lamp 85. Therefore, the above method causes problems that it shows a low manufacturing efficiency and a low productivity, which will be overcome by the present invention.